This invention relates to a coupling of the blind screwdown type in which two objects are fastened together by engaging and screwing together a female coupling member onto a protruding male coupling member, each being provided with screw threads.
In the particular environment in which the preferred embodiment of the invention resides, the prior art device is a high precision oil filter having a removable filter insert disposed in an annular space between a cylindrical outer casing and a central pipe member.
The filter is provided with a removable lid to permit access to the filter elements for their inspection and replacement as needed during the life of the filter. The lid must, of course, be sealed against the pressure of the fluid being filtered, typically oil. For this reason, the lid is normally provided with precision gaskets to be compressed under pressure against the outer cylindrical surface and some means of fastening, against internal pressure, the lid to the cylindrical filter body.
Any fastening means chosen must be usable by typical operational personnel or semi-skilled mechanical personnel who in the field normally are the persons who inspect and change the oil filter at intervals. It is found that this type of servicing task is typically handled by semi-skilled personnel, and any fastening means chosen must be safe, effective and reliable when used by persons of little intrinsic skill. Further, the method of fastening must be intuitively obvious, as the typical servicing person cannot be expected to either be highly trained in the operation of the filter or to exhibit a great deal of skill in determining whether the filter is properly reassembled.
For this reason, the filter fastening means chosen is almost inevitably some form of heavy threaded screw fastener. A first typical type of fastening is the use of a hinge and wing nut permitting the lid to be tilted open or fastened down by a wing nut on a threaded fastener. For a number of reasons, including possible loss of the wing nut, substitution of unsuitable substitute nuts, and the difficulty of insuring adequate torque against the internal pressure, this is a less successful expedient, and the preferred method of fastening involves screwing the lid on a threaded central post within the filter so as to provide a symmetrical, uniform hold down force.
This type of fastening, however, is a blind fastening. The lid obscures the vision of the engagement of the screw thread and the central pipe and the engagement of the threads is typically by feel. Since the intuitive response of a person engaging two threads to fasten an object together is to tighten the object until a certain amount of resistance (back torque) is felt, and then assume that the fastening has been correctly engaged provided the lid does not easily pull off, there is a high potential for cross threading. As one example, filter inspection by factory personnel of returned filters of the type used for the preferred embodiment indicated that substantially all the filters showed evidence, particularly scarring around the fastening handle base by pipe pliers, of excessive torques being applied to overcome a cross threaded fastening and secure the filter lid. Inspection of the internal fastening threads likewise indicated a substantially high incidence of cross threaded engagement which is destructive to the threads and ultimately destructive to the filter.
Furthermore, repeated cross threading of a filter weakens the joint to the point where actual filter lid failure can occur causing a possible explosion of the filter, sudden loss of filtered oil and resulting severe damage to the object whose oil is being filtered. Since the typical large oil filter is utilized on unattended continuously running machinery, such as diesel engines and the like, the potential for an undetected sudden filter failure, with resulting loss or destruction of the prime mover poses an unacceptable risk.
Previous attempts to provide a form of alignment between the mating male and female threaded fasteners for blind engagement have proven relatively unsuccessful; these include attempting high precision machining of the lid to provide for high tolerance alignment of the joint; providing a V-grooved angular ring circumferentially around the outer edge of the lid so that the lid is brought into identical alignment with the cylindrical outer body of the filter, and chamfering and counter-sinking the female fastening to provide a guide for the male fitting.
In each case it has been found that, in any structure that provides a suitable amount of freedom of motion of the female fitting so as to align and secure the lid and provide sufficient freedom of engagement, cross threading still occurs with an unacceptable degree of frequency.